Group III nitrides such as gallium nitride (GaN) based semiconductors are expected as a material for next generation power semiconductor devices. A GaN-based semiconductor has a wider bandgap compared with silicon (Si). For this reason, GaN-based semiconductor devices can realize higher breakdown voltage and lower loss than silicon (Si) semiconductor devices.
Generally, in a GaN-based transistor, a high electron mobility transistor (HEMT) structure using a two dimensional electron gas (2DEG) as a carrier is applied. A normal HEMT is a normally-on transistor which conducts without a voltage applied to the gate. Accordingly, there is a problem that it is difficult to realize a normally-off transistor that does not conduct unless a voltage is applied to the gate.
A power supply circuit, or the like, that deals with large electric power of several hundred V to 1000 V demands normally-off operation with emphasis on safety. Accordingly, a circuit configuration that realizes normally-off operation by cascode-connecting a normally-on GaN-based transistor with a normally-off Si transistor is proposed.
However, such a circuit configuration includes a problem of element breakdown and characteristic degradation when an over voltage occurs at a connecting portion between the two transistors.